The invention relates to a process for winding a film web onto a winding core to form a film roll in contact with a contact cylinder, via which the film web is fed to the film roll, with the contact cylinder moving oppositely to the contact roll at the same circumferential speed. The invention also relates to a device for winding a film web.
A process of this type and a device for winding up a film web are known from DE-A1-3,710,412 and work satisfactorily, for example, for winding speeds of the film webs up to approximately 100 m/min with very thin film webs, for example 3 .mu.m PET films (PET=polyethylene terephthalate).
In the production of films, the winding of the film web as the last step of the process assumes considerable importance, because, during this, a film web which is manufactured true to specification can be wound "incorrectly" in such a way that there can be a total loss of the film roll due to damage of the film web resulting from winding tensions which are too high.
A problem which is especially important in this respect is the phenomenon that considerable quantities of air are also included in the film roll during the winding of the film web. Some of this included air escapes from the film roll during its storage. Thereby, various defects, such as cave-ins, stretching and transverse corrugations, can occur in and on the film roll and can cause the film roll to be completely useless.
In a known device for winding sheet-like structures, the film web is fed onto a roll having a shaft which is motor-driven. The film web is pressed against the roll by means of a so-called contact roller, thus ensuring that smaller quantities of air are included than without this measure. This device is described in German Patent 3,265,570 which corresponds to U.S. Pat. No. 4,576,344. At higher winding speeds, i.e., at speeds above 100 m/min, the air-displacing effect of the contact roller is no longer sufficient to displace a sufficient quantity of air entering the roll, and therefore there has to be a compromise here between the winding parameters and the inclusion of air. The entire phenomenology of air inclusion, with special consideration of the dependence of the air inclusion on the pressing force of the contact roller against the roll and of the requisite winding tension in view of the physical properties of the film, is the subject of research and development.
It has also already been proposed to carry out the entire winding operation in a vacuum, in order thereby to solve the problem of the inclusion of air, but the high financial outlay is an obstacle to putting this proposal into practice.